Eric J. Jokela, Wendell P. Cropper Jr., Salvador A. Gezan. School of Forest Resources and Conservation, University of Florida, P.O. Box 110410, Gainesville, ...
Forest Ecology and Management 342 (2015) 84–92
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Variation in biomass distribution and nutrient content in loblolly pine (Pinus taeda L.) clones having contrasting crown architecture and growth efficiency Angelica M. Garcia Villacorta, Timothy A. Martin ⇑, Eric J. Jokela, Wendell P. Cropper Jr., Salvador A. Gezan School of Forest Resources and Conservation, University of Florida, P.O. Box 110410, Gainesville, FL 32611-0410, USA
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Article history: Received 21 November 2014 Received in revised form 14 January 2015 Accepted 16 January 2015
Keywords: Ideotype Crown structure Allocation Biomass distribution
a b s t r a c t Loblolly pine (Pinus taeda L.) is well adapted across an extensive range of sites, is responsive to silvicultural treatments, and has undergone genetic improvement through traditional tree breeding programs, with selection based primarily on rapid growth and disease resistance. Loblolly pine clones with contrasting crown architecture provide an opportunity to better understand the mechanistic relationships among stem growth, biomass partitioning, and component nutrient content. We assessed inventory data from four clones which exhibited a range of crown widths. Using a subset of three clones, we measured crown sizes, crown volume, biomass allocated to components (foliage, branches, stemwood and bark), and component nutrient concentration and content to assess variation in allocation and growth efficiency. Clonal variation in biomass distribution patterns helped explain variation in growth efficiency between the narrow crown clone (ARB-1) and wide crown clone (ARB-4). Clone ARB-1 was more efficient at producing stem biomass increment per unit foliar biomass and unit foliar nutrient content than clone ARB-4; this was consistent with the concept of a crop ideotype. This study provides new information useful for improving our understanding of the relationships among crown structure, biomass distribution patterns, growth efficiency, and tree productivity, and may help guide clonal tree population management. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The development of intensive management practices in forest plantations in the southeastern United States has made the region the woodbasket of the world (Wear and Greis, 2002). Improvements in silvicultural technologies such as fertilization and competition control have greatly increased the productive potential of forest stands (Fox et al., 2007). Genetic improvement of loblolly pine (Pinus taeda L.) has also significantly contributed to large increases in plantation productivity, producing a wide variety of highly productive open-pollinated (half-sib) and full-sib families and, more recently, clones (McKeand et al., 2006; Fox et al., 2007). Deployment of trees in clonal blocks (‘‘clonal forestry’’) has a number of potential benefits compared to more diverse genetic deployments, including increased stand uniformity, increased gain in growth and disease resistance, and improved wood quality (Bettinger et al., 2009). Few published studies have examined tree- or stand-level characteristics of clonal stock, however, and further research is needed to better understand the ⇑ Corresponding author. Tel.: +1 352 846 0866. E-mail address: tamartin@ufl.edu (T.A. Martin). http://dx.doi.org/10.1016/j.foreco.2015.01.012 0378-1127/Ó 2015 Elsevier B.V. All rights reserved.
dynamics of clonal stands compared to more traditional deployments (McKeand et al., 2003; Emhart et al., 2007; Aspinwall et al., 2011b) A potentially useful approach for assessing clonal phenotypes is the ideotype model proposed by Donald (1968) for agronomic crops. Donald and Hamblin (1976) proposed three classes of ideotypes: isolation ideotypes that perform best in young stands when intraspecific competition is minimal; competition ideotypes that grow well through aggressive competition with their neighbors and are less efficient users of site resources (Cannell, 1978); and crop ideotypes that achieve high productivity through efficient use of resources and are less aggressive intra-specific competitors. It is reasonable to hypothesize that crop ideotypes should be ideal for intensively managed production systems (Dickmann, 1985). Ideotypes offer a valuable model for evaluating variation in crown traits and tree and forest productivity (Dickmann et al., 1994 Nelson and Johnsen, 2008). It has been proposed that crop ideotypes would have more compact, narrow crowns, while competition ideotypes would have wider crowns (Cannell, 1978, Martin et al., 2005). Within the context of stand dynamics, crown traits may influence competitive interactions among trees. Many crown architectural traits are under genetic influence in southern
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pines, and therefore may be incorporated into breeding programs if desired (McCrady and Jokela, 1998; Emhart et al., 2007). Previous research has shown that soil nutrient availability is the primary factor limiting growth in southern pines in their native range (Fox et al., 2007). Previous studies with loblolly pine (McKeand et al., 2006; Tyree et al., 2009b, Stovall et al., 2011) and other tree species have shown genetic variation in nutrient traits (Beets and Jokela, 1994; Gonzalez and Fisher, 1997; Xu et al., 2003). How nutrient traits interact with crown traits and efficiency of growth has not been examined, however. Typically, genetic experiments comprise single-tree plots or row plots not using operational management or fully representing ecosystem dynamics (Martin et al., 2001). Physiological variation and stand dynamics in larger, genetic block-plot studies may offer useful information for understanding genotype performance under operational conditions (Martin et al., 2005). The present study examines variation in tree- and stand-level stem growth, biomass partitioning, and component nutrient content in loblolly pine clones having contrasting crown architecture. We hypothesized that narrow-crowned clones would have greater stand-level efficiency in terms of stem biomass produced per unit foliage or biomass nutrient content, consistent with predictions for crop ideotypes. The study used the Varietal Architecture Investigations Examining Tree Interactions on Experimental Sites (VARIETIES) clonal block plot experiment installed near Starke, Florida. The goal of this study was to increase our understanding of the relationship between crown architecture and tree productivity, and to explore the causes of growth efficiency variation. 2. Materials and methods 2.1. Study area and experimental design VARIETIES is a clonal block plot experiment established in 2009 near Starke, Florida (29°560 5000 N, 82°60 2900 W). The study was established as a split-plot, randomized complete block experiment with four replicates. The whole-plot treatment was planting spacing (wide, 2.7 m 3.7 m, 1000 trees/ha, and narrow, 1.5 m 3.7 m, 1802 trees/ha), with split-plots planted with genetic entries of four clones (ARB-1, ARB-2, ARB-3, ARB-4) and one full-sib family. An additional genetic entry consisted of the four clones planted in mixture. The different genetic entry split-plot sizes were 7 7 tree plots (280 m2 for and 490 m2 for narrow and wide spacing, respectively). In this study, only the clonal genotypes growing in pure plots were analyzed, and all biomass sampling and scaling up calculations were performed only on the wide planting spacing treatment. 2.2. Inventory measurements An age 3 year inventory conducted in December, 2011 included measurements of tree height (H, m), diameter at breast height (DBH, cm), crown length (CL, m) and crown width (CW, m) measured in two directions for all genotypes in the study. Stem volume index (SV = DBH2 ⁄ H, dm3), crown volume (CV, m3 and approximated as a paraboloid), relative crown width (CW/H, m/m) and an index of stem volume growth efficiency (SV/CV, dm3/m3) and crown shape ratio (CL/CW, m/m) were derived from primary measurements. 2.3. Biomass sampling Based on an analysis of the inventory data, three contrasting clones were chosen for measurement of biomass distribution: ARB-1, ARB-2, and ARB-4. ARB-1 and ARB-4 had the narrowest
and widest average CW/H, respectively, and also had contrasting SV/CV (Table 1). In addition, we chose to measure biomass distribution for ARB-2 because this clone had the greatest productivity of all clones in the study (Table 1). For each clone, we chose eight trees distributed across the full range of tree DBH for destructive harvest. Harvested trees were chosen from among all four replicate plots for each clone in the wide planting spacing treatment. The destructive harvest was carried out on March 13 and March 16– 19, 2012. Each sample tree was cut at the base and separated into stem and branch + foliage components. Branches with foliage attached (foliated branches, FB) and branches without foliage attached (NFB) were weighed separately. The total green weight of stem, branch, and branch + foliage components was measured in the field. Subsampling in the field enabled scaling of field green weights to component dry weights as follows. Subsample disks of stem were weighed green in the field, separated into stem wood and bark components which were also weighed green, and then bagged for laboratory drying. Non-foliated branch subsamples were weighed green in the field and then bagged for laboratory drying. Foliated branch subsamples were weighed green, separated into branch and foliage components that were also weighed green, then bagged for laboratory drying. Subsamples were then dried in the lab at 70 °C to a constant weight. Due to field and/or data recording errors, data were discarded for two trees from each of clones ARB-1 and ARB-4. As a result, the final sample size for clones ARB-1, ARB-2, and ARB-4 was 6, 8, and 6 trees, respectively. Additional subsamples of stem wood, bark, branch, foliated branches, non-foliated branches, and foliage were collected from a small, medium, and large biomass harvest tree from each clone. The subsamples were chipped and ground in a Wiley Mill to pass through a 1 mm stainless steel sieve. The subsamples were sent to an independent laboratory (Micro–Macro International, Athens, Georgia, USA) for nutrient analysis as follows. About 0.5 g of ground tissue samples was first dry ashed in a muffle furnace and then the samples were brought up to volume with aqua regia (3:1 HNO3: HCl). The extracts were then analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES; MMI Labs, Athens, GA, USA). Total N was analyzed in a CNS analyzer (LECO Corporation, St. Joseph, MI, USA) using the Dumas Method. 2.4. Data analysis Data from destructively harvested trees were used to create allometric functions describing distributions of stem wood, stem bark, branches, and foliage for each of the three clones. Allometric relationships were fitted by regressing age 3 year individual tree component biomass against total individual tree biomass using a log–log regression approach. The generic model was:
lnðyÞ ¼ b0 þ b1 x þ e;
Table 1 Age 3 year tree-level means from analysis of variance for stem volume (SV, dm3), crown volume (CV, m3), crown width (CW, m), relative crown width (CW/H, m/m) and stem volume growth efficiency (SV/CV, dm3/m3) for four clones in the VARIETIES experiment near Starke, FL. Clone
SV*
CV
CW
CW/H
SV/CV
ARB-4 ARB-3 ARB-1 ARB-2 p-Value
15.32 b 12.44 b 14.34 b 24.22 a
